Field of the Disclosure
The present disclosure relates to a non-reciprocal circuit element, and particularly relates to a non-reciprocal circuit element such as an isolator or a circulator used in the microwave bands.
Description of the Related Art
Hitherto, a non-reciprocal circuit element such as an isolator or a circulator has characteristics of transmitting signals only in a predetermined specific direction and not transmitting signals in the opposite direction. By utilizing the characteristics, for example, the isolator is used for a transmitting circuit unit of a mobile communication apparatus such as a cellular phone.
As such a type of non-reciprocal circuit element, an isolator is known in which, as shown in FIG. 9, on a microwave magnetic material 32 (hereinafter, referred to as ferrite) to which a DC magnetic field G is applied by a permanent magnet, a first center electrode 35 (inductor L1) and a second center electrode 36 (inductor L2) are disposed so as to intersect each other in an insulated state, one end of the first center electrode 35 is an input port P1, the other end of the first center electrode 35 and one end of the second center electrode 36 are an output port P2, a terminal resistor R is connected between the input and output ports P1 and P2 and in parallel with the first center electrode 35, and a capacitance element C11 is connected between the input and output ports P1 and P2 and in parallel with the terminal resistor R. In addition, a capacitance element C2 is connected between the output port P2 and a ground port P3 and in parallel with the second center electrode 36.
The isolator has low insertion loss and is able to operate in a wide band. Patent Document 1 describes a non-reciprocal circuit element that basically has such a circuit configuration to allow isolation characteristics to be adjusted, without deterioration of insertion loss, by making the capacitance element C11 as a variable capacitance element or using a switching element to make a resonance capacitance variable.
Meanwhile, the magnetic permeability of the ferrite and the magnetic force of the permanent magnet have unique temperature characteristics. It is possible to adjust the isolation characteristics of the ferrite and the permanent magnet at normal temperature, but the isolation characteristics are not able to completely follow the characteristics variation caused due to temperature change, and the isolation characteristics and the insertion loss characteristics vary from set values. FIG. 10 shows an example of the characteristics variation caused due to the temperature characteristics of the ferrite and the permanent magnet. In FIG. 10, curves A and A′ indicate the isolation characteristics and the insertion loss characteristics at 25° C., curves B and B′ indicate the isolation characteristics and the insertion loss characteristics at 85° C., and curves C and C′ indicate the isolation characteristics and the insertion loss characteristics at −35° C. That is, in the example shown in FIG. 10, the isolation characteristics shift to the high frequency side when the temperature becomes higher than 25° C., which is normal temperature, and shift to the low frequency side when the temperature becomes lower than 25° C. In addition, in the example shown in FIG. 10, when the temperature becomes higher or lower than 25° C. which is normal temperature, the insertion loss characteristics also vary in accordance with the change in temperature.
The reason why the isolation characteristics deteriorate is that, due to the temperature characteristics of the ferrite and the permanent magnet, the effective values of the inductors L1 and L2 change, so that the resonant frequencies of an LC resonant circuit composed of L1 and C11 and an LC resonant circuit composed of L2 and C2 vary. To prevent this, it is necessary to prepare a magnet and a ferrite having temperature characteristics adjusted to a wide band, or to adjust the capacitance value of the resonant capacitance element C11 such that variation of the resonant frequency is cancelled. However, it is very difficult to prepare materials having temperature characteristics adjusted to the wide band. In addition, temperature detection with high accuracy and capacitance control with respect to the temperature are required for the adjustment of the capacitance element C11, which is not practical.
Patent Document 1: International Publication No. 2012/020613